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Methane is the second-most important greenhouse gas responsible for global warming, after carbon dioxide. Emissions have been surging since 2007, causing a spike in atmospheric concentrations.

Finding out where that methane comes from is tricky. Potential sources include microbes in natural wetlands, rice paddies, landfill sites and the stomachs of ruminants such as cows, emissions from coal mines, and leaks from gas wells and pipelines – with natural gas largely composed of methane.

Now, Stefan Schwietzke at the US National Oceanic and Atmospheric Administration in Boulder, Colorado, believes he and his team have cracked the problem by analysing the isotopic make-up of methane in thousands of air samples from around the globe. Microbial emissions are rich in the isotope carbon-12, whereas fossil-fuel sources are rich in carbon-13.

The researchers found that methane emissions from fossil fuels are at almost twice the level previously estimated – at some 200 million tons per year – but have not been rising over time.

“Despite the large increase in natural gas production, there has not been an upward trend in industrial emissions,” says Schwietzke, attributing this to major advances in plugging leaks at wells and in pipelines.

Instead, the team says, the recent methane spike has resulted from a so-far unexplained surge in microbial emissions, especially in the tropics.

Euan Nisbet at Royal Holloway, University of London, thinks that microbes in natural wetlands and rice paddies may be releasing more methane because of increasing rainfall or temperatures. This could be, he says, “a troubling harbinger of more severe climate change”, triggering a runaway rise in methane emissions.

Underestimates

The finding that the fossil fuel industry’s claims about the size of its methane emissions are serious underestimates mean that many national emissions inventories submitted to UN climate negotiators may be wrong.

The team’s estimate that fossil fuel emissions are twice as big as thought included figures from natural geological seepage of gas. With those excluded, the estimates for the fossil fuel industry alone are still between 20 and 60 per cent greater than inventories suggest.

“The natural gas industry is becoming much better at controlling gas leaks,” says Nisbet. “But much more can be done. It would be one of the most cost-effective ways of cutting greenhouse warming.”